Implements alpha blending for RT-Thread GUI 0.8

In RT-Thread GUI 0.8, we implement anti-aliasing drawing line with:
rtgui_dc_draw_aa_line/rtgui_dc_draw_aa_lines.

alpha blending drawing line and filling rect with:
rtgui_dc_blend_line/rtgui_dc_blend_lines;
rtgui_dc_blend_fill_rect/rtgui_dc_blend_fill_rects.